Variability and climate of brown dwarfs and exoplanets

Work at the Observatoire du Mont-Mégantic made it possible to detect the signature of clouds on the surface of a cold brown dwarf for the first time. Brown dwarfs have relatively short rotation periods, on the order of several hours. If clouds are distributed irregularly on the surface of a brown dwarf, it is possible to observe periodic variations in brightness as it rotates.

Figure 1. SIMP0136+0933 flux during 4 nights over 1 week. Variability is clearly apparent, a sign of the presence of clouds on the surface of this 900 °C brown dwarf.

By observing the brown dwarf SIMP0136+0933 for several nights, an object with a temperature of 900 °C, we managed to detect a variation in its flux every 2.4 hours, indicating the presence of clouds. In addition, the form of the light curve evolved from one night to another (see Figure 1), suggesting that the cloud models at the surface of SIMP0136+0933 change shape after several days. This discovery, published in 2009, led researchers to look for similar variability on other objects, in collaboration with researchers from Toronto and elsewhere (e.g. 12). These results allowed us to determine that the variability of cold brown dwarfs comes from breaks in the dust clouds (titanium oxide, iron hydride), revealing occasional much warmer and dust-free regions beneath the clouds.

This type of phenomenon has now been observed on giant planets, and it is probable that instruments on board the JWST telescope will make it possible to detect the signature of cloud patterns on the surface of giant planets like those of HR8799.